A crane is conventionally provided with a winch device for performing hoisting operations (crane operations). A winch device is provided with a winch drum that rotates by being driven by a motor, and performs hoisting or lowering of an object by means of this winch drum. The hoisting or lowering of the object is instructed by an operator operating an operation lever. In the winch device, the motor performs hoisting or lowering of the object by causing the winch drum to rotate in a hoisting direction or lowering direction, in accordance with the operation of the operation lever.
Furthermore, the winch device is provided with a negative brake. The negative brake holds the winch drum so that the winch drum cannot rotate by applying a braking action to the winch drum, when the operation lever is disposed in a neutral position, and releases the braking action applied to the winch drum when the operation lever is operated from the neutral position. However, in a winch device of this kind, there is a possibility that the timing at which the negative brake releases the braking action applied to the winch drum may diverge from the timing at which the motor starts rotation of the winch drum. If the timing of the release of the braking action applied to the winch drum is earlier than the timing of the start of rotation of the winch drum, then a slipping-down phenomenon occurs in which the object drops momentarily. If the timing of the release of the braking action applied to the winch drum is later than the timing of the start of rotation of the winch drum, then a pull-up effect occurs in which the winch drum performs a hoisting operation of the object while the braking action is still applied to the winch drum by the brake.
The following Patent Documents disclose technology for preventing the occurrence of phenomena such as the foregoing.
Patent Document 1 describes a winch braking device provided with a pilot valve that generates a pilot pressure corresponding to the operation of an operation lever from a hydraulic pressure supplied from a pilot hydraulic pressure pump, a direction control valve that controls the direction of rotation of a hydraulic motor in accordance with the generated pilot pressure, and a hydraulic brake which applies a braking action to the winch drum. A switching valve and a brake value are arranged in series between the pilot hydraulic pressure pump and the hydraulic brake. The switching valve is switched to a state for supplying hydraulic pressure from the pilot hydraulic pressure pump to the brake valve, by the pilot pressure generated by the pilot valve in accordance with the operation of the operation lever. The brake valve is switched to a state of supplying hydraulic pressure from the switching valve to the hydraulic brake in accordance with hydraulic pressure supplied from the direction control valve to the hydraulic motor. The hydraulic brake applies a braking action to the winch drum when no hydraulic pressure is supplied from the brake valve, and releases the braking action on the winch drum when hydraulic pressure is supplied from the brake valve. Consequently, when the hydraulic motor starts rotation of the winch drum due to hydraulic pressure being supplied from the direction switching valve to the hydraulic motor, then the braking action of the hydraulic brake on the winch drum is released simultaneously due to hydraulic pressure being supplied from the brake valve to the hydraulic brake. As a result of this, the slipping-down phenomenon and the pull-up phenomenon described above are prevented.
Furthermore, in Patent Document 2, a counterbalance valve is provided in the piping that supplies hydraulic pressure to the hydraulic motor. A pressure sensor for detecting the holding pressure inside the piping is provided in the piping between the counterbalance valve and the hydraulic motor. When the operation lever is in the neutral position, a braking action is applied to the winch drum by the brake device. When the operation lever is operated to the hoisting side, then the brake device is controlled so as to gradually reduce the braking force on the winch drum, until the holding pressure detected by the pressure sensor reaches a target holding pressure required in order to hold the suspended load. Therefore, sudden release of the holding of the suspended load, and the subsequent occurrence of a slipping-down phenomenon, is prevented. Furthermore, when the holding pressure detected by the pressure sensor has risen to the target holding pressure, then the brake device is controlled in such a manner that the braking force on the winch drum immediately becomes zero. Therefore, the occurrence of a pull-up phenomenon of the brake is prevented.
In recent years, there have been demands for electrification of winch devices used in cranes. In an electric winch device, a winch drum is caused to rotate by using an electric motor instead of a hydraulic motor, and an object is hoisted by the winch drum, but an electric winch device of this kind also has the problems of slipping-down and pull-up phenomena described above. However, in order to prevent the occurrence of these phenomena in an electric winch device, it is difficult to apply the technologies disclosed in the Patent Documents indicated above. More specifically, the technologies in the Patent Documents indicated above use hydraulic pressure which is supplied to a hydraulic motor in order to control a brake releasing operation, and cannot be applied to an electric winch device which does not have hydraulic pressure of this kind.